Peroxide-curable fluoroelastomers derived from units of vinylidene fluoride and a fluorine-containing monomer that is copolymerizable with vinylidene fluoride, such as hexafluoropropylene or pentafluoropropylene, optionally containing tetrafluoroethylene and up to 3 mole percent of units derived from a bromine-containing olefin can be peroxide cured to produce fluoroelastomers having good physical properties and exhibit good resistance to steam degradation. These fluoroelastomers that employ a bromine-containing olefin as a cure site ordinarily contain a processing aid; otherwise, it is not only difficult to remove the cured fluoroelastomer from the mold cavity, but it is frequently damaged when removed. Also, without processing aids it is difficult to obtain an extruded article having a smooth surface. However, processing aids have an adverse effect on the physical properties of the cured fluoroelastomer. Most significant changes are a reduction in tensile strength of the fluoroelastomer and a lessening of compression set resistance, a most important property of fluoroelastomers, especially when the elastomer is used as a gasket or O-ring. The present invention provides a fluoroelastomer composition comprising an elastomeric copolymer of vinylidene fluoride, a terminally ethylenically unsaturated fluoromonomer, a bromine-containing olefin, and a particular sulfone that functions as a processing aid in the peroxide-curable composition. The resulting fluoroelastomers have the advantages of similar compositions containing processing aids but, at the same time, the processing aid used in the present invention does not adversely affect the physical properties of the cured fluoroelastomer but quite surprisingly, actually maintains or improves the tensile strength and the compression set resistance of the fluoroelastomer at various temperatures.